How the central nervous system of invertebrates handles the staggering number of mechanical variables involved in even the simplest movement is one of the central problems in motor control. Even a simple task, like a reaching movement of the arm toward an object involves the simultaneous activation of many thousands of motor units belonging to a number of muscles. The complexity inherent in controlling so many degrees of freedom has led the investigator and a number of other researchers to propose that the nervous system must have developed ways to simplify this problem. In the last few years this investigative team has asked a specific question: is the motor behavior of vertebrates based on simple units (motor primitives) that can be flexibly combined to accomplish a variety of motor tasks? They have addressed this fundamental and long-standing question by utilizing spinalized frogs and rats. With an array of approaches such as microstimulation of the spinal cord, NMDA iontophoresis and cutaneous stimulation of the hindlimb, they have provided evidence for a modular organization of the frog's and rat's spinal cord. A module is a functional unit in the spinal cord that generates a specific motor output by imposing a specific pattern of muscle activation. In the work covered by this proposal the investigators plan to investigate five key questions related to modularity: 1. They intend to ascertain whether the modules they have identified in the spinal cord are utilized by supraspinal and reflex pathways to generate postures and movements. 2. They will investigate whether there is a common set of muscle synergies subserving different motor behaviors. 3. They will record the activity of spinal interneurons to understand the physiological substrate of the spinal modules. 4. They will investigate the dynamic properties of the spinal modules. 5. Through stimulation studies, they will establish the relation between the modules' mechanical properties and their competence to generate postures and movements.